Photometric absorption detectors are used for determining the optical density of fluid and solid materials as a function of constant or variable light wave lengths. This permits, for instance, the determination of the presence and quantity of substances in a sample, in particular in a chromatographically separated sample in the process of liquid chromatography.
In the process of liquid chromatography, the components of a test mixture leave the separating column in the solvent flow in a chronological order of zones and are fed through a capillary tube into the photometric absorption detector. Given a specific concentration, a homogeneous distribution and a pre-determined layer thickness, the optical density of the individual components is dependent upon the light wave length (absorption characteristics). If the highest possible sensitivity is to be reached in determining any given component of the test mixture, the measurement of the optical density must be carried out with the wave length set to the absorption maximum. On the other hand, it may in certain cases be desirable to suppress any given component in the chromatogram. In this case, the wave length must be set to an absorption minimum for the component in question.
However, there are certain difficulties encountered in adapting the measuring wave length during a chromatogram to the absorption characteristics of the respective components, as in certain separating processes, the time interval between the components leaving the column is often limited to a few seconds only. It is difficult and sometimes even impossible to adapt the wave length in a time as short as this, the more as the moment of the adjustment must be very exactly defined, too. This difficulty is still aggravated by the fact that the absorption detector must be re-adjusted following each adjustment to bring the output signal as exactly as possible to a level equal to that given before the adjustment, in order to avoid discontinuities of the recording. In addition, certain cases reqire also an adjustment of the sensitivity of the detector in order to maintain a uniform format for the peaks of different components in the chromatogram.
A further problem of the known absorption photometers, spectral photometers and photometric detectors for liquid chromatography lies in the fact that the base line is instable, i.e. that the output signal of the detector does not remain constant during times in which no sample is in the measuring cell. In liquid chromatographs, such trouble is mainly caused by the flow of the solvent through the very small measuring cell, but also by intensity fluctuations of the light source.
It is true that interfering influences to the output signal may be reduced by the particular optical construction and by the stabilizied operation of different components, such as light source and photo-detectors. However, in the case of detector arrangements for liquid chromatographs, which are required to exhibit a very high base line stability, it is very difficult to get the interfering influences under control. Moreover, it is a fact that in liquid chromatographs with only one flow channel from the sample input to the detector, the interferences resulting from the solvent flow cannot even be efficiently suppressed by a detector with two optical channels. For, in this case, the reference measuring cell must be inserted into the flow channel before the feed-in point of the sample, and as a result the interferences arising from the solvent flow will appear in the two measuring cells with a certain time lag, so that it will be impossible to achieve a complete compensation.